This invention relates to an engine starter apparatus and, more particular, to an engine starter apparatus provided with a planetary gear speed reduction mechanism.
An engine starter apparatus having a planetary speed reduction gear is known as disclosed in Japanese Patent Laid-Open No. 63-90665, for example. The engine starter apparatus disclosed in this Laid-Open is the so-called coaxial starter apparatus in which a solenoid switch is disposed behind a d.c. motor, and the planetary speed reduction gear 1 comprises, as illustrated in the FIGURE, a sun gear 4 formed on the outer circumference of the front end of an armature rotary shaft 3 of a d.c. motor 2, an internal gear 6 formed in the inner circumferential surface of a machine frame 5 disposed outside of the sun gear 4, three planet gears 7 rotating between the sun gear 4 and the internal gear 6, and a carrier 8 rotatably supporting and connecting each of the planet gears 7.
In the illustrated planetary speed reduction gear 1, when the numbers of teeth of the sun gear 4, the internal gear 6 and the planet gear 7 are Zs, Zi and Zp, respectively, the relationship between the number of teeth of each of the gears 4, 6 and 7 is determined so that they satisfy the equation of (Zs+Zi)/N=an integer (where, N is the number of the planet gears 7), which is the engagement condition for the plurality of planet gears, and the equation of Zi=Zs+2Zp, which is the engagement condition for a single planet gear, as well as the range of Zs.gtoreq.11 and Zi.ltoreq.46, with the module M=1.25 for the reasons of mechanical strength of the armature rotary shaft 3 on which the sun gear 4 is formed and the installation lay-out of the starter apparatus with respect to the engine. This relationship can be expressed in a table shown in Table 1 in conjunction with various combinations of gears 4, 6 and 7 when Zs is 11, Zp is 16, 17 or 18, and Zi is one of 41-49.
TABLE 1 ______________________________________ Planetary Speed Numbers of Reduction (Zs + Zi)/N = Teeth Ratio Integer No. Zs Zp Zi Zs + 2Zp - Zi g N = 3 ______________________________________ 1 11 16 41 +2 4.73 X 2 " " 42 +1 4.82 X 3 " " 43 0 4.91 O 4 " " 44 -1 5.0 X 5 " " 45 -2 5.09 X 6 " 17 43 +2 4.91 O 7 " " 44 +1 5.0 X 8 " " 45 0 5.09 X 9 " " 46 -1 5.18 O 10 " " 47 -2 5.27 X 11 " 18 45 +2 5.09 X 12 " " 46 +1 5.18 O 13 " " 47 0 5.27 X 14 " " 48 -1 5.36 X 15 " " 49 -2 5.45 O ______________________________________
As apparent from Table 1, the gear arrangement that satisfies the previously discussed conditions is the third combination, in which Zs is 11, Zp is 16 and Zi is 43, and therefore the conventional planetary speed reduction gear 1 is arranged according to this combination, so that the planetary speed reduction ratio g obtained by g=1+(Zi/Zs) is 4.91.
Demands for miniaturizing various car equipments is increasing in view of reduction of weight and efficient use of the space within the engine compartment of a vehicle, and the engine starter apparatus is not an exception. One measure for miniaturizing the engine starter apparatus includes miniaturization of the d.c. motor, which may be realized by increasing the planetary speed reduction ratio g. In this case, from the previous equation g=1+(Zi/Zs), either the number of teeth Zs of the sun gear 4 can be decreased or the number of teeth Zi of the internal gear 6 can be increased.
However, since the number of teeth Zs of the sun gear 4 and the number of teeth Zi of the internal gear 6 is determined according to the previously described restriction, these numbers of teeth of the gears 4 and 6 cannot be changed under the condition that at least the performance of the conventional engine starter apparatus be maintained (not decreasing the speed reduction ratio), so that there was a problem that the miniaturization of the d.c. motor by increasing the planetary speed reduction ratio g is very difficult.